First, pray.
Second, pray.
Third, pray.
Then…in my case, I spent 30-plus years in the evangelical Protestant world (raised Catholic). I know the mindset, the theological issues, and the answers. Some of my wife’s family, evangelical Protestant, are a bit bewildered by my return. I have had to defend some of our doctrines from Scripture. I always go with Scripture because they will accept nothing else.
I also venture out onto other online Forums, evangelical Protestant, and debate theological issues.
And then, in my neighborhood here in NYC, I know a few pastors since I was starting to train to become a minister and did a little preaching (at Salvation Army and a Lutheran congregation). I still see them and have had to defend our beliefs that differ with theirs, always from Scripture.
And then, there is a local Methodist congregation that has a weekly Bible study/faith sharing group. I attended before my return and stayed, when I wasn’t busy in my own parish. I have shared my return to the Church with them, and have to speak out sometimes but I am very aware that this is not my church and they have a right to their beliefs. But there are a lot of Catholics there.
I have fellowship with two other Catholic men who were also away from the Church for many years. One was converted outside the Church like me, and the other was converted through AA and the Cursillo movement. We all have the same desire…to bring Catholics back to Jesus in His Church.
I hope this helps a bit. If you feel called to speak with Catholics who have become Protestants, you will have to bulk up on apologetics and Scripture. For me, it was no problem because I always spent a lot of time reading systematically through the Bible, reading books, listening to sermons and radio shows. And I also started to take some pre-seminary courses in hermeneutics, preaching, etc.
Peace,
Gene